A spectroscopic measurement is used as a long-term stable and reproducible measuring principle in, for example, carbon dioxide sensors. The radiation of an infrared source in the MIR (mean infrared range) on a typical CO2 absorption wavelength (e.g., 4.3 μm) is compared with the radiation of the infrared source for a reference wavelength (e.g., 4 μm). Information on whether and in what concentration CO2 is present in the absorption path between the radiation source and the detector results from the comparison of the radiation intensity for the absorption wavelength and the reference wavelength.
Because of economic reasons, incandescent lamps, the irradiation spectrum of which reaches into the MIR range, are frequently used as the radiation source. The main limitation for the usability of the lamps is the MIR absorption of the lamp glass which limits the utilization of the incandescent lamps to the IR wavelength range<4.5 μm. Special glass qualities also absorb a large portion of the IR radiation in the wavelength range of just 4 μm. Due to the irradiation maximum of the incandescent filament of the lamp in the range of visible light (wavelength<1 μm), the largest portion of the generated radiation additionally represents a power loss which may also have a detrimental effect on the measuring accuracy.
Ceramic infrared radiators, special incandescent lamps made of silica glass, or devices known in the laboratory field as black-body radiators may represent alternatives to an incandescent lamp as an infrared source. If one intends to detect other gases, such as CO for example (absorption at 4.6 μm), using the above-mentioned spectroscopic measuring principle, one has to resort to these IR sources which are much more expensive and more complex.
An electrically operable tubular infrared radiator which is situated in a reflector is described in German Patent document No. 198 12 188. The ceramic infrared radiator has a carrier tube made of sintered Al2O3 and a heater coil made of resistor wire which is wound on the carrier tube, the resistor wire being enclosed and held by a cover layer made of a ceramic compound which, at least on its surface, is dyed black. The reflector is designed as a molded body made of ceramic material having a focal line, the reflector being gold-plated on its side facing the carrier tube including the heater coil. The carrier tube including the heater coil is situated in the area of the reflector's focal line.
A carbon dioxide sensor having a substrate carrier with a heating element including power terminals attached to its bottom side is described in detail in German Patent document No. 44 37 692. Interdigital electrodes with a carbon dioxide-sensitive material on top are situated on the top side.